1. Field of the Invention
The present invention relates to a transmitter and a receiver, and more particularly to a transmitter and a receiver capable of controlling a packet retransmission request and a packet retransmission decision in accordance with the type of data to be sent, as well as the data string to be sent, in a packet retransmission request for a packet loss detected in a receiver, and in a transmitter retransmission decision for the retransmission request.
2. Description of the Related Art
In recent years, the transmission of image data, voice data and other such data has been energetically carried out via various communications media, such as Internet communications. In Internet-based data transmissions in particular, in addition to the download-type transmission systems, which have been utilized for some time, there has been an increase in services based on stream-type transmission systems.
A stream-type transmission system is one in which the reproduction of image and voice data received by a receiving side terminal is executed in parallel with the carrying out of image and voice data transmission from a transmitting side to a receiving side terminal, and is being used in Internet telephony and remote teleconferencing. As the transmission protocol best suited to this kind of stream-type transmission system, UDP/IP (User Datagram Protocol/Internet Protocol) and the like are typically being used. When data is transmitted in packets using this UDP/IP transmission protocol, since the transmitting side specifies the destination IP address and transmits unilaterally, if packet loss should occur along the transmission channel, the transmitting side has no way of knowing whether or not the packets arrived at the destination, and the receiving side has no way of knowing if all of the intended packets from the transmitting side arrived, which means that lost packets are not automatically retransmitted.
Further, as the UDP/IP host protocol there is the IETF RFC1889-specified protocol called RTP (Real-time Transport Protocol). If data transmission is carried out in accordance with the RTP/UDP/IP protocols, since reproduction-related time information and packet number information is attached to each packet, the temporal relationship of the data being sent can be discerned on the receiving side by referencing this time information and packet numbers, making it possible to carry out synchronized reproduction.
In a data transmission in accordance with the RTP/UDP/IP protocols, when packet loss is detected at the receiving side, a retransmission request is sent to the transmitting side, and the lost packet is sent once again. However, if the receiving side performs a retransmission request for all lost packets, and the transmitting side performs a retransmission for all retransmission requests, efficiency drops, and the available transmission band comes under pressure. Thus, efficient retransmission control is desirable.
To solve for the above problem, a method has been proposed, whereby the packet type (I-picture, P-picture, B-picture, and so forth) is determined, and a retransmission request is only sent when the packet type is determined to be an I-picture, as disclosed in Japanese Laid-open Patent No. 11-98503. Further, as indicated in Japanese Laid-open Patent No. 2001-274681, a method has been proposed, whereby, when the transmitting side receives a retransmission request, the transmission number of each I-picture is set at 1, the precedence of the nth picture from this I-picture is set at 1/n, and retransmission is not carried out for a picture having a precedence of a certain threshold value or less. Now then, when packet loss occurs and a retransmission request is issued during the transmission of image data, in the method whereby a retransmission request is only made when the packet type is determined to be an I-picture, image degradation due to packet loss can be held down to a certain extent, while reducing retransmission requests as much as possible by virtue of the fact that a retransmission request is issued only for the I-picture, which is the most important packet type.
However, when packet loss occurs for P-picture as well, conventional control is not always sufficient since an error is propagated in the subsequent frame. In other words, if there is no I-picture for a long time after a P-picture that experienced packet loss, an accurate display cannot be made until the next I-picture appears, thus prolonging the effects of this error. Further, this same problem occurs even with a method whereby the precedence is set in advance by a function, and retransmission is only carried out when this precedence is not less than a threshold value.